Photocatalytic dehydrogenative C-C coupling of acetonitrile to succinonitrile

The coupling of acetonitrile into succinonitrile, an important terminal dinitrile for value-added nylon production, via a dehydrogenative route is highly attractive, as it combines the valuable chemical synthesis with the production of green hydrogen energy. Here, we demonstrate that it is possible to achieve a highly selective light driven dehydrogenative coupling of acetonitrile molecules to synthesize succinonitrile using anatase TiO2 based photocatalysts in aqueous medium under mild conditions. Under optimized conditions, the formation rate of succinonitrile reaches 6.55 mmol/(gcat*h), with over 97.5% selectivity to target product. Mechanism studies reveal that water acts as cocatalyst in the reaction. The excited hole of anatase semiconductor oxidizes water forming hydroxyl radical, which subsequently assists the cleavage of sp3 C-H bond of acetonitrile to generate ·CH2CN radical for further C-C coupling. The synergy between TiO2 and Pt cocatalyst is important to enhance the succinonitrile selectivity and prevent undesirable over-oxidation and hydrolysis. This work offers an alternative route to prepare succinonitrile based on renewable energy under mild conditions and avoid the use of toxic reagents and stoichiometric oxidative radical initiators.

tape and folded to get sufficient signals. The data pretreatment and fitting of EXAFS spectra were performed using Ifeffit packages 2 .

Transmission electron microscopy (TEM)
The TEM and HADDF-STEM images of catalysts were performed with a JEOL JEM-2100F instrument at an acceleration voltage of 200 kV.

UV-Vis Spectroscopic characterization
UV-vis diffuse reflectance spectra (UV-vis DRS) were obtained on a Shimadzu DUV-3700 spectrophotometer equipped with an integrating sphere attachment.
Transient photocurrent 3 The 5mg sample was mixed with 0.5ml ethanol solution for ultrasonic treatment for 1 hour, and then the prepared sample was dropped onto ITO conductive glass of 1 cm × 2 cm. The sample was dried in the oven at 60℃ to ensure that the sample would not fall off. The glass was used as the working electrode. The electrolyte solution was 1 M Na2SO4 solution with reference electrode of mercurous sulfate and counter electrode of Pt. The electrochemical workstation was manufactured by Shanghai Chenhua Instrument Co., LTD. The test method was i-t Curve with a sensitivity of 1.e-6. During the test, the conductive glass stained with the sample was inserted into the electrolyte solution and fixed in a suitable position. Then turn on the light (with a 365nm filter) and test for 20s. Repeat the above steps 5 times to obtain the photocurrent spectrum.

Photoluminescence spectroscopy
Horibalabram-hr confocal laser microraman spectrometer produced by JY, France, was used for PL spectrum. The excitation light source was HE-Cd laser (365nm) and the grating was 2400.

Fluorescence probe experiment
Photocatalyst powder (10 mg) was suspended in a Pyrex cell containing 10 mL of a 0.1 mM coumarin aqueous-CH3CN solution, and one side of the cell was irradiated with the LED beam. The suspension was stirred vigorously for 20 min before and during the light irradiation at 60 °C. After the irradiation, the clear solution was taken out and its fluorescence spectrum was measured by the Fls1000 fluorescence spectrophotometer with the excitation wavelength at 332 nm.

Photocatalytic Reaction Test.
Photocatalytic experiments (acetonitrile dehydrogenative coupling to succinonitrile) were performed in a top-irradiation Pyrex flask. A 10W LED light (wavelength 365 nm) (PLS-SXE300, Beijing Trusttech Co., Ltd.) was used as the light source. Typically, 20 mg photocatalysts were dispersed in 10 mL 70% volume acetonitrile aqueous solution under magnetic stirring. Prior to the irradiation, the reaction mixture was deaerated repeatedly with Ar gas for 5 times to thoroughly remove air and dissolved oxygen. During the reaction, the photocatalytic reaction system was kept at 60℃. To evaluate the photocatalytic hydrogen production and analyze other gas products, the gas-phase composition of the photocatalytic reactor was analyzed by an Agilent 8860 gas chromatograph equipped with 5 Å molecular sieves and HP-Plot columns and thermal conductivity cell (TCD) detector. Liquid products were analyzed by Agilent 8860 gas chromatograph equipped with a column of SH-1 with flame ionization detector (FID). The formation rate of all products, average turnover frequency (ATOF) and carbon-based selectivity of products were defined in equation (1)

AQY calculation methods.
In generally, producing one · CH2CN radical from CH3CN needs one hole with one electron consumed at the same time (e.g., one SN molecule needs two electrons). Then the AQY could be calculated according  The thermodynamic evaluation of the CH3CN dehydrogenative coupling reaction in Figure S1 suggests that the dehydrogenative coupling of acetonitrile is thermodynamically unfavourable reaction, which cannot occur spontaneously at ambient condition.

Supplementary Discussion
The photoluminescence spectra in Supplementary Figure 8

Supplementary Discussion
The fluorescence probe method was used to detect the hydroxyl radical desorbed from the catalyst surface after radiation. The coumarin (408 nm) was used as the probe which would capture the free •OH and transform into umbelliferone (445nm). Significant amount of free •OH radicals was observed after 20 min radiation at 60 °C, suggesting the coupling reaction occurred both in the solution and on the surface of catalysts.